Generally, photolithography techniques used in the fabrication of semiconductor devices utilize an imaging system that directs radiation onto a mask to form a pattern. The pattern is projected onto a semiconductor wafer covered with light-sensitive photoresist. Once exposed, the photoresist material may be developed to remove excess photoresist material. The remaining photoresist material acts as an etching mask for an etching process used to pattern the underlying semiconductor wafer.
Ongoing improvements in lithography have allowed the size of semiconductor integrated circuits (ICs) to be reduced, thereby allowing devices with higher density and better performance. One highly promising lithography system uses radiation in the extreme ultraviolet (EUV) wavelength range. Generally, EUV lithography (EUVL) uses radiation having wavelengths of about 10 to 15 nm located between the soft x-ray and the vacuum ultra-violet (VUV) wavelength range.
Generally, EUVL imaging systems are reflective systems. EUV reflective systems, which may be used as an illuminator, projection optics, reflective optics, condenser optics, reflective photo masks, or the like, use multi-layer, thin-film coatings known as distributed Bragg reflectors. The multi-layer coatings typically comprise 40-70, or more, Mo/Si bi-layers with the bi-layer thickness being about half of the respective EUV wavelength being used.
During use, however, the surface of the EUV reflective optics including reflective masks which are also considered to be optical elements frequently become contaminated. Surface oxidation and carbon deposits are particularly troublesome and can shorten the useful life of the EUV reflective optics such that the use of EUV reflective optics is not commercially feasible. Carbon deposits occur due to the absorption of CH-containing molecules (hydrocarbons) on the optics surface from residual gases in the vacuum environment or absorption of carbon containing molecules (CO, CO2) and subsequent photon- or secondary electron induced dissociation and desorption reactions. Resist outgassing may also lead to carbon deposition on the mirror surfaces through photodissociation or through electron-induced dissociation by photon generated secondary electrons of hydrocarbons. Surface oxidation may result from residual water vapor through absorption of water and subsequent photon-induced or secondary electron-induced dissociation of H2O where the oxygen remains on the surface and the hydrogen desorbs.
Carbon contaminates may be removed reversibly by controlled introduction of oxidizing gases such as H2O. However, the partial pressures of hydrocarbon-containing gases and water vapor pressure must be tightly controlled within a very small process window that prevents oxidation without leaving too much carbon on the surface. The process is further complicated because EUV optics in an EUV exposure tool are exposed to different EUV intensities and the process window may be different for each mirror. Furthermore, during EUV exposures oxidation is enhanced by generation of highly reactive radicals (e.g., O, OH) via dissociation of gas phase molecules by the intense EUV radiation above the mirror surfaces. The generation of radicals may be different for different light intensities and, therefore, for different mirrors.
Attempts have been made to solve this oxidation problem by adding capping layers of silicon, ruthenium, and layers of silicon and ruthenium modified by adding oxygen and/or nitrogen over the surface of the EUV optics. However, it has been found that these capping layers were not oxidation resistant and did not provide an effective barrier layer against diffusion of oxides, e.g., O2, O, OH, or the like, such that the capping layers were penetrated by these molecules and/or atoms which then oxidized the multi-layer stack below the capping layer causing changes of the optical constants and the thicknesses of the individual layers.
Therefore, what is needed is an EUV optics structure that resists contamination by carbon and oxidation.